Abstract: In this paper, the equivalent conversion principle from self-balanced Q—S curve to traditional Q—S curve on pile head is introduced simply. Reliability of test data is analyzed and randomization of each parameter is considered. Distribution of each parameter is obtained. Particularly, the method how to obtain distribution of every factor from the test data which are affected by two random factors and the statistical method how to use test data obtained in different times (influencing factors of each time are different) to perfect the scanty stylebook(two data in this paper) are discussed. Reliability of self- balanced test method is analyzed at last.